1. Field of the Invention
The present invention relates to a raw alloy for a permanent magnet for use in various types of motors, meters, sensors and loudspeakers, for example, and a method of making such a raw alloy. More particularly, the present invention relates to a raw alloy for a nanocomposite-type permanent magnet to be prepared by a strip casting process and a method of making such an alloy.
2. Description of the Related Art
A nanocomposite-type permanent magnet, having a structure in which a hard magnetic phase such as R2Fe14B and soft magnetic phases such as Fe3B and α-Fe (i.e., high-magnetization ferromagnetic phases) are magnetically coupled together, is now under development as an R—Fe—B based magnet. A powder of a nanocomposite-type permanent magnet is compacted into a predetermined shape with a resin material, thereby forming an isotropic bonded magnet.
In producing a nanocomposite magnet, a rapidly solidified alloy, having either an amorphous structure or at least a structure consisting mostly of an amorphous phase, is often used as a start material thereof. When subjected to a heat treatment, this rapidly solidified alloy is crystallized and eventually becomes a magnetic material having a nanocrystalline structure with an average crystal grain size of about 10−9 m to about 10−6 m.
The structure of the heated and crystallized magnetic alloy heavily depends on the structure of the rapidly solidified alloy that is yet to be heated and crystallized. For that reason, to obtain a nanocomposite magnet having excellent magnetic properties, it is important how to define the conditions of rapidly cooling and solidifying a melt of the raw alloy because those conditions should determine the specific structure (e.g., the percentage of amorphous phase) of the resultant rapidly solidified alloy.
A rapid cooling process to be performed with a machine such as that shown in FIG. 1 is known as a conventional method of preparing such a rapidly solidified alloy including a greater volume percentage of amorphous phase. In this process, a molten alloy is ejected out of a nozzle, having an orifice at the bottom, toward a rotating roller made of copper, for example, and rapidly cooled by the roller, thereby obtaining a thin-strip amorphized solidified alloy.
Methods of this type have been researched and reported by universities and organizations that are engaged in the study of magnetic materials. However, a machine for use in those researches or reports is modeled just for experimental purposes so as to melt several tens to several hundreds grams of alloy inside of a nozzle and eject it out of the nozzle. That is to say, a machine having that low processing rate cannot mass-produce a raw alloy for a nanocomposite magnet.
Thus, a method of achieving increased processing rates is described in Japanese Laid-Open Publications No. 2-179803, No. 2-247305, No. 2-247306, No. 2-247307, No. 2-247308, No. 2-247309 and No. 2-247310, for example.
In this method, a molten alloy, which has been melted in a melting crucible, is poured into a container having an ejecting nozzle at the bottom, and then ejected out of the nozzle by applying a predetermined pressure onto the melt in the container (this method will be referred to herein as a “jet casting process”). By ejecting the melt through the nozzle while applying a pressure thereto in this manner, a stream of the melt (or a melt flow) having a relatively high flow rate can be ejected substantially perpendicularly toward around the top of the rotating roller. The ejected melt forms a puddle (i.e., a melt puddle) on the surface of the rotating roller. A portion of this puddle, which is in contact with the roller, is rapidly cooled and solidified, thereby forming a thin-strip rapidly solidified alloy.
In the jet casting process described above, the molten alloy and the rotating roller have just a short contact length. Accordingly, the melt cannot be rapidly cooled and solidified completely on the rotating roller, and the alloy at a high temperature (e.g., 700° C. to 900° C.) is still cooled and solidified even after having peeled off the rotating roller and while traveling in the air. In the jet casting process, the cooling process is carried out in this manner, thereby amorphizing any of various types of alloys.
In the jet casting process, however, if the processing rate is increased to an industrially mass-producible level (e.g., about 1.5 kg/min or more), the ejecting nozzle is worn out significantly with an increase in the feeding rate of the melt (or the melt ejecting velocity). As a result, the melt feeding rate is subject to change during the process, and the constant rapid cooling state cannot be maintained anymore. In addition, a huge nozzle cost should be needed in that case. Furthermore, since the melt ejecting velocity is limited by the nozzle diameter, it is difficult to increase the processing rate as intended.
Also, in the jet casting process, the melt might be solidified at the nozzle and possibly clog the nozzle up. Accordingly, a mechanism for keeping the container with the nozzle at a predetermined temperature is needed. Furthermore, to keep the melt ejecting rate constant, a mechanism for controlling the pressure on the surface of the melt inside the container and the pressure on the outlet of the nozzle is also needed. As a result, the initial equipment cost and equipment operation cost are both expensive.
Furthermore, to constantly achieve an appropriate cooling rate for an amorphous alloy by the jet casting process, the melt ejecting velocity needs to be relatively low (e.g., 1.5 kg/min or less). Thus, the jet casting process is not so productive. However, if the melt ejecting velocity is too high in the jet casting process, then no puddle might be formed on the surface of the roller but the melt might splash off, thus making the melt cooling rate inconstant.
Also, in the jet casting process, a rapidly solidified alloy including a greater volume percentage of amorphous phase is obtained by ejecting a small amount of melt onto a roller that rotates at a relatively high velocity (e.g., at a peripheral velocity of 20 m/s or more). Thus, the resultant thin-strip rapidly solified alloy typically has a thickness of 50 μm or less. It is difficult to collect a thin-strip alloy having such a small thickness so efficiently as to increase the tap density thereof sufficiently.
On the other hand, a strip casting process is also known as another method of preparing a rapidly solidified alloy. In the strip casting process, a molten alloy is supplied from a melting crucible onto a shoot (or tundish) and then brought into contact with a chill roller, thereby making a rapidly solidified alloy. The shoot is a melt guiding means that controls the flow rate of the melt by temporarily reserving the melt thereon and rectifies the melt flow, thereby feeding the melt onto the chill roller constantly and continuously. The melt, which has come into contact with the outer circumference of the chill roller, moves along the circumference of the roller so as to be dragged by the rotating chill roller and cooled in the meantime.
In the strip casting process, the melt and the outer circumference of the roller have a relatively long contact length in the circumferential direction of the roller. Thus, the melt can be cooled and solidified substantially completely on the roller.
As described above, the strip casting process uses no ejecting nozzle unlike the jet casting process but feeds the molten alloy continuously onto the rotating roller by way of the shoot. Thus, the strip casting process is effective for mass production and can reduce the manufacturing cost.
In the strip casting process, however, a large amount of molten alloy is fed onto the roller and the rapid cooling rate tends to be low. For these reasons, the strip casting process is not effective for preparing an amorphized solidified alloy. If the rapid cooling rate is low, then an alloy including a smaller volume percentage of amorphous phase (i.e., including a greater volume percentage of crystalline structure) is formed easily. If there is a greater volume percentage of crystalline structure in the alloy structure, the crystalline structure will grow excessively from the crystalline nuclei in the subsequent heat treatment for crystallization. As a result, a nanocomposite magnet having excellent magnetic properties cannot be obtained.
For these reasons, the strip casting process is often used to make completely crystallized flakes of a metal (see Japanese Laid-Open Publication No. 8-229641, for example). A rapidly solidified alloy obtained in this manner is normally used as a raw alloy for a sintered magnet including an R2Fe14B phase as its main phase, and cannot be used as a raw alloy for a nanocomposite magnet.
As described above, it has been difficult to make a raw alloy, including a greater volume percentage of amorphous structure, for a nanocomposite magnet highly productively and at a reduced cost.
In order to overcome the problems described above, an object of the present invention is to provide a raw alloy for a nanocomposite-type permanent magnet having excellent magnetic properties at a reduced cost.